U.S. Pat. No. 3,612,664, to which German 17 69 123 corresponds, describes a method and a device for substance separation from a liquid mixture by means of fractional crystallization. In this method the liquid mixture is flowing in turbulence over indirectly cooled crystallization surfaces, and the crystal layer, after reaching a certain thickness, is removed by melting from the crystallization surface. For this prior art layer crystallization method, a crystallizer is used in which the vertical cooled, or in special cases heated walls are provided by a number of vertical tubes arranged in parallel. The liquid mixture flows inside of the tubes from the top to the bottom in form of an thin film, whereas on the outside of the tubes the coolant is present. The coolant flowing as a thin film on the outside of the crystallizer tubes provides, by undercooling, local crystallizing centres in the thin film flowing on the inner wall. The local crystallizing centres grow together to a continuous layer. From time to time, when the deposited crystal layer reaches a certain thickness, it is melted. This method has the disadvantage that it is necessarily discontinuous, and therefore, after each cycle, the whole device must be heated and cooled. This requires a substantial amount of energy and increases the cycle time. Normally, the described method is carried out in different stages, because the purity of the crystallized product is relatively limited due to the high temperature gradients and the small substance transfer areas. The cited literature proposes to carry out fractionating in cycles of different steps or stages. As melting liquid for the crystals from stage N-1 the mother liquor of the stage N+1 of the previous cycle is used, and the mother liquor of stage N is used as melting liquid for the crystals of stage N-2 of the following cycle. This method became known as the Sulzer-MWB crystallization method (see S. J. Jancic, "The Sulzer-MWB Fractional Crystallization System", Sulzer Technical Review, April, 1988).
The U.S. Pat. No. 3 603 103 discloses a method in which crystals produced by a cooler provided with a scraping mechanism are fed into the middle of a column. In this column the crystals and the liquid mixture to be purified, often also called melt, are fed in counter-current. The temperature of the melt is controlled in such a way that the crystals are growing on their passing through the column. At the end of the column the crystals are melted. The melted crystals are partly removed as final product and partly returned in counter-current to the growing crystals in the column. At the other end of the column the highly impure rest, the so-called mother liquor, is withdrawn. This method has the advantage that it is continuous. The counter-current provides intensive substance exchange. However, this method hat the disadvantage that strong axial mixing takes place. Accordingly, the separation effect obtained by crystallization is substantially reversed. The growing crystals transport adhering melt, and the melt flowing in counter-current transports small crystals.
The European published application 0 167 401 shows a crystallization device for separating or purifying organic substances. This crystallization device comprises three vertical concentric hollow cylinders. Between the inner and the middle hollow cylinder is a first interspace for receiving a cooling fluid. Between the middle and the exterior hollow cylinder is a second interspace for receiving the melt to be purified. The outside of the middle hollow cylinder forms the crystallizing surface. The first interspace has an inlet and an outlet for the cooling fluid. The second interspace has an inlet for the melt to be purified and an outlet for the melted crystallization product, that is the pure product, and an outlet for the impure product. Diametrically opposite to the inlet for the melt to be purified a barrier is located in the second interspace to prevent the flowing-through of melted crystallized product. An electric heater is used to melt the crystallized product. In operating the crystallizing device the crystallizing surface rotates slowly in direction to the outlet for the pure product. On this path a crystal layer is formed on the crystallizing surface. This crystal layer is melted by the electric heater. Depending on the setting of the outlet valve, more or less of the melted crystallization product is flowing through the outlet, whereas the rest flows back in the second interspace toward the inlet, is mixed with the melt to be purified, and flows further toward the outlet for the impure product. On this path, crystals are further deposited on the crystallization surface. Because on this path the melt contains less and less material crystallizable at a temperature of the crystallization surface, hardly any crystallization takes anymore place near the outlet for the impure product. At the outlet for the impure product a valve is provided. Further, a pump is provided to backfeed, if desired, a part of the impure product to the inlet for further crystallization.
The described crystallization device has, in particular, the disadvantage that the cooling liquid in the first interspace provides over the whole crystallization surface is practically at the same temperature. Accordingly, already in case of a relatively thin thickness of the crystallization layer practically no crystallization takes place anymore. Therefore, the yield remains relatively small. A further disadvantage is that the speed of backflow of the melted crystallized product is not everywhere the same, because of friction, faster cooling at the edges, and flow of a partial stream through the outlet. Further no controlled sweating is possible.
The published European patent application 0 063 688 discloses a crystallizing device which, in particular, can be used for undercooled melt. The device comprises a precrystallization device and a horizontal cooling band and has the purpose to provide an optimum nucleation speed and a high nucleation number also without the necessity of adding additional seeding material to the precrystallization device. In the precrystallization device nucleation is initiated in a thin film, and crystallization takes place on the cooling band to which the precrystallized melt is applied in a thick layer. This crystallization device was created especially for the solidification of undercooled melts, and the patent application does not contain any suggestion how a substance separation by crystallization could take place with the device shown.
The French patent 1 220 220 discloses a number of different crystallization devices, one of them having an inclined crystallizing surface. This crystallizing surface is formed by one side of a length of an endless band. Under this length of band is a cooling device, and above the length of band is a feeding station for feeding the liquid mixture on the crystallizing surface. This device has the disadvantage that the inclination of the inclined crystallizing surface can not be changed. A further disadvantage is that the cooling device has no freely selectable temperature profile over the length of the crystallizing surface. Also no return line is provided to permit washing of the crystal layer.